[mythtv] DVB support - Status update
linux.news at bucksch.org
Thu May 1 02:18:37 EDT 2003
Andy Davidoff wrote:
> I hope to write a panacea tuning patch in another couple weeks, so
> if you can do a good job of describing the semantics of DVB tuning,
> I'll try to give you some suggestions for generally applicable
DVB tuning itself is not *all* that complicated.
First, you have 4 DVB "carriers" (not sure what the official term is): S
(sat), C (Cable), T (terrestrical=antenna) and (hardly used) IP
(Internet). Each of those have different ways to "tune" to a given
transport stream. DVB-S has frequency, polarization (horizontal or
vertical) and symbol rate. DVB-T has frequency, modularizaion, bandwidth
and a number of other parameters (5-10) needed to tune. Not sure about
DVB-C, but I think it only needs the frequency.
DVB-S has a special parameter, which I guess is uncommon these days,
namely diseqc, it controls the motors used to rotate the sat dish,
allowing you to receive from any satelite. Of course, once you move the
dish, you change it for *all* connected cards. It also takes some time
to move the dish (a few secs, I think). Not sure, if we want to support
that at all, it sure can drawf a whole design.
Secondly, there are several program streams in a transport stream. A TS
consists out of any number of streams, each assigned a PID. A TV
"channel" usually consists out of one stream for video, one for audio
and maybe others for other data (teletext, metadata etc.). One TS
typically (depending on the bitrate) contains 5-8 TV channels. So, you
need to know which PIDs a channel has, in addition to the other tuning
parameters above. When a DVB receiver tunes, it receives a full TS at
once. The "full" DVB cards have "demultiplex" hardware, which you tell a
number of PIDs and the hardware demuxes the TS, leaves out everything
apart from the streams with those PIDs, remuxes that and feeds it to
you, so you get a single TV channel only from the card. The "budget"
cards, however, have the ability to give you (the application) a full
TS. The application (or driver) is then responsible to to the demux and
remux/conversion to get to a single channel. I wrote my code so that we
can exploit that and receive the same stream several times, each time
filtering out other program streams. Long story short, you can, with a
single DVB card, receive several TV channels at once, if (and only if)
they happen to be on the same TS. You know that they are, if all the
DVB-type-specific tuning parameters I described above are identical and
only the PIDs differ.
An implementation gotcha here is that the "recorder" (the part of Myth
that reads the data from the device) needs to know the PIDs to be able
to filter for them, not just the "channel" (the part of Myth that
tunes). (I have solved that with the current code, the recorder asks the
channel for the PIDs upon recording start and the channel notifies the
recorder of PID changes upon channel change.)
Of course, you have on top of that all the problems caused by PayTV.
DVB-cards can decrypt internally using a special hardware interface, but
that only works on the card where the smartcard is connected to. So, you
may have channels available on one card, but not the other, while both
cards can receive all FreeTV channels.
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